Aqueous MgCl.sub.2 solutions are obtained by various techniques, such as by aqueous chlorination of Mg-containing minerals (e.g., dolomite), by taking natural MgCl.sub.2 brines from natural underground deposits, or by chlorination of Mg hydrate which is formed by alkalizing seawater. These MgCl.sub.2 brines are useful for various purposes, such as being a source of MgCl.sub.2 for use as cell feed to an electrolytic cell where molten MgCl.sub.2, dissolved in a molten salt electrolyte, is electrolyzed to form Mg metal at the cathode.
One of the common impurities found in MgCl.sub.2 brines is Ca.sup.++, present as a soluble compound.
Various techniques have been used to substantially reduce the level of Ca.sup.++ ions in MgCl.sub.2 brines, such as by treatment with excess sulfate ions to suppress solubility of the Ca.sup.++ as CaSO.sub.4 which is only slightly soluble in water (whereas MgSO.sub.4 is highly soluble); after separating the insolubles there still remains a small amount of Ca.sup.++ in solution and there still remains a need for sulfate removal from the MgCl.sub.2 solution. There is a need for reducing the Ca.sup.++ content of relatively concentrated MgCl.sub.2 solutions in an efficient and expeditious manner, especially MgCl.sub.2 solutions which are intended to be the source of MgCl.sub.2 as a cell feed to an electrolytic Mg cell.
In the Journal of The American Chemical Society, Vol. 78, number 23, Dec. 8, 1956, pp. 5963-5977, there are two companion articles by D. W. Breck, et al. The articles are: Crystalline Zeolites I. The Properties of a New Synthetic Zeolite, Type A, and Crystalline Zeolites. II. Crystal Structure of Synthetic Zeolite, Type A. These published articles are incorporated herein by reference to show the preparation and analyses of Type A zeolites. These Type A zeolites are said to be represented by the formula Na.sub.12 [(AlO.sub.2).sub.12 (SiO).sub.12 ].27H.sub.2 O, characterized as cubic, a.sub.0 =12.32 A., space group O.sub.h '-Pm3m, having a 3-dimensional network consisting of cavities 11.4 A. in diameter separated by circular openings 4.2 A. in diameter. Removal of the crystal water leaves a stable crystalline solid containing mutually connected intracrystalline voids amounting to 45 Vol. % of the zeolite. The article also discloses that the sodium ions undergo cation exchange in aqueous solution and that for dipositive ions the order of decreasing selectivity of the Type A zeolite was determined to be EQU Zn&lt;Sr&lt;Ba&lt;Ca&lt;Co&lt;Ni&lt;Cd&lt;Hg&lt;Mg
and in Table VII of the article it is disclosed that the selectivity of Ca.sup.++ is 0.72 and of Mg.sup.++ is 0.43, these materials being each tested as 0.2 N solutions at 25.degree. C. The selectivity x (100) is defined as the extent of cation exchange achieved by contacting the zeolite with a solution containing the exact equivalence of exchanging ion.
In the intervening years since the above articles were published, the sodium forms of the synthetic Type A zeolites, conforming substantially to the empirical formula EQU Na.sub.12 [(AlO.sub.2).sub.12 (SiO.sub.2).sub.12 ].nH.sub.2 O
have become known in industry as Type 4A zeolites and are sold as such by Linde Air Products Company, Davison, and others. It is these Type 4A zeolites which are of interest in the present invention.
We have found that in highly concentrated MgCl.sub.2 brines that a zeolite of the 4A structure, Na.sub.12 [(AlO.sub.2).sub.12 (SiO.sub.2).sub.12 ].nH.sub.2 O, has an unexpectedly high selectivity for calcium ions with respect to the much higher concentration of magnesium ions. As pointed out above, the selectivities of Mg.sup.++ and Ca.sup.++ were tested by Breck, et al at 0.2 N concentrations. With our instant solutions the Mg is on the order of 25 times (or more) the 0.2 N concentration used by Breck, et al. The selectivity as listed by Breck, et al shows zeolite Type 4A to prefer Ca:Mg at a ratio of 72:43 when contacting the zeolite with an exact equivalency of exchanging ions. We have now found that the zeolite, 4A, when challenged by the high-Mg.sup.++, low-Ca.sup.++ concentrations still rejects Mg.sup.++ and removes Ca.sup.++ to low levels.
Stated another way, one would expect from Breck, et al that with a concentrated MgCl.sub.2 brine containing a small percent of Ca.sup.++ ions, the high concentration of Mg.sup.++ ions would override the tendency toward Ca.sup.++ selectivity and that the Mg.sup.++ selectivity would prevail in such a high concentration.
We have unexpectedly found that the present process, which uses a Type 4A zeolite, selectively removes low concentration Ca.sup.++ ions from a concentrated MgCl.sub.2 brine. The sodium ions which are substituted for the calcium are then readily removed as NaCl which precipitates when the brine is further concentrated. The zeolite can readily be returned (regenerated) to the sodium form with a concentrated NaCl brine.